Herein, a brake is a device for arresting the motion of a mechanism by friction, wherein the mechanism can be any wheeled vehicle such as a car, plane or train. A brake component is any component whose primary function is to cause the frictional force or transfer of said force to arrest the motion of a mechanism. Examples of the brake component include a torque tube, piston housing, rotor and stator, described hereinbelow. A friction element is a brake component in which at least a portion contacts another friction element causing a frictional force to be developed. Illustrative examples of a frictional element include a brake pad and a rotor or stator wherein the rotor or stator directly contact another friction element.
Because of the demands of flight, the materials used to construct aircraft brakes have to meet more stringent criteria compared to other vehicles such as automobiles. In an aircraft, there are three basic modes of brake operation: normal service (landing), rejected take off (RTO) and taxi stops and snubs (taxiing). Upon landing a commercial aircraft such as a Boeing 737, the brake parts which provide friction to arrest motion (friction element) typically heat up to a temperature of 600.degree. C. to 800.degree. C. An RTO is the most severe brake operation, wherein the wear rate of the brake can be a thousand times or more greater than a landing and the temperature of the frictional material of the brake can easily exceed a temperature of 1000.degree. C. or more. After an RTO, the brake is scrapped because the conditions are so severe. Taxi snubs and stops occur when the plane taxis to and from the runway. A snub is when the plane does not come to a full stop. Because aircraft require large amounts of braking energy in short periods of time (i.e., seconds), the friction element should have as large a specific heat as possible, wherein specific heat is the quantity of heat required for a one degree temperature change in a unit weight of material. Also, the friction element should have a low density to decrease aircraft weight and, subsequently, increase payload or decrease fuel consumption.
There are essentially two types of aircraft brakes in service today. The first type is a steel brake. The second type is a carbon/carbon composite brake. Each aircraft brake type has a brake assembly typically comprising a hydraulic piston assembly, torque tube, torque plate, integral wheel and alternating rotors and stators. The torque tube is typically made of steel or a titanium alloy. The wheel and hydraulic piston assembly are typically made of an aluminum alloy.
Typically, the aircraft brake assembly is configured as follows. The torque tube has grooves on the outer diameter running longitudinally the length of the tube to a flange. Typically, a backing plate (flat disk having an outer and inner diameter) is first slid onto the torque tube outer diameter until contacting the flange. The rotors and stators are then alternatingly slid onto the torque tube outer diameter. The rotors and stators are disks also having an inner and outer diameter. The rotors and the backing plate have no grooves on the inner diameter to engage the torque tube but have grooves or mounting means on the outer diameter to attach to the inner diameter of the wheel. The stators have grooves on the inner diameter which engage the torque tube. A pressure plate (a disk having inner diameter grooves engaging the torque tube) is then slid onto the torque tube. On top of the pressure plate is attached the hydraulic piston assembly which is connected to the torque tube by inner diameter grooves or by bolting to the torque tube. The above assembly is then slid over a landing strut axle and the torque tube is mounted to the landing strut at the hydraulic piston assembly end.
The wheel is attached to the backing plate and rotors of the above assembly. The wheel is typically attached by grooves on the inner diameter of the wheel which engage grooves on the outer diameter of the backing plate and rotors. The wheel is mounted to the axle by bearings and thrust nuts.
Functionally, the rotors spin with the wheel until application of the piston to the pressure plate, wherein the rotors contact the stators. Upon rotor-stator contact, torque is created by friction between the rotors and stators. The torque is transmitted to the landing strut via the torque tube, thus slowing the wheel and aircraft. The rotor-stator contact results in wear of the rotors and stators and also in significant heat generation. The stack of rotors and stators are commonly referred to as the heat sink because this is the part of the brake that absorbs energy, converts it to heat and then dissipates it to the atmosphere.
Steel brakes have pairs of rotors and stators, as described above, in which steel rotors (friction element) typically carry the brake pads and the stator is comprised of high-strength, high temperature steel. In a steel brake, the friction elements are the brake pads and stator. The brake pads which contact the stator are typically a metal matrix composite (MMC) wherein the matrix is copper or iron. The pads can be bonded to a rotor or stator by brazing, welding, riveting or direct diffusional bonding. The brake pads, typically, are in the form of segmented pads of some geometry such as trapezoids uniformly positioned around the face of the rotor or stator.
The second type of brake is a carbon/carbon composite brake. Carbon/carbon composite brakes have rotors, stators, backing plate and pressure plate made out of carbon/carbon composite. In this brake, the rotors and stators are the friction elements. Typically, a carbon/carbon composite is a composite of continuous carbon filaments embedded in a carbon matrix. The properties of the composite can vary widely depending on the processing and filament orientation.
As aircraft get ever bigger and faster, the amount of energy necessary to stop an aircraft during landing and RTO continues to increase. These two trends have necessitated the decrease of weight wherever possible and required the brakes to handle ever increasing energy inputs into the heat sink of the brake. Loads have increased because the size of the wheels and, hence, brakes are limited (i.e., by design and weight considerations). Because of weight, steel brakes, in general, are not used on larger commercial aircraft such as the Boeing 747.
Because carbon/carbon composites have a density of about a quarter of the density of steel, carbon/carbon composite brakes are generally used in high speed military aircraft and large commercial aircraft today. However, carbon/carbon composites have a specific heat (e.g., J/K-g) that is only about two times greater than the specific heat of steel. Thus, a carbon/carbon composite brake would have to be at least twice the size of a steel brake if limited to the same temperature increase as a steel brake during a landing or RTO. Carbon/carbon composite brakes avoid this unacceptable increase in size by operating at significantly higher temperatures than steel brakes. The higher temperature at which a carbon/carbon composite brake can operate is limited by the ability of surrounding structures (e.g., hydraulic piston assembly, wheel and tire) to withstand the temperature generated by the carbon/carbon heat sink and by the tendency of the carbon/carbon composite to oxidize at higher temperatures which causes unacceptable wear.
The coefficient of friction (.mu.) of a friction material is desirably as great as possible. The coefficient is desirably as great as possible to minimize the load that is necessary to generate the frictional force (frictional force=.mu..times.normal load) needed to stop a plane. Carbon/carbon composites tend to adsorb water, which decreases the coefficient of friction. The lowered coefficient of friction lasts until the brake has heated up sufficiently during braking to evaporate the water.
During braking, the coefficient of friction of a carbon/carbon composite friction material may vary by a factor of 3 or more causing a corresponding torque variation which can lead to undesirable vibration. Carbon/carbon composite also displays a static coefficient of friction that is less than the dynamic coefficient. This frictional behavior may cause problems during stopping due to the increased load necessary as the wheel slows down.
Two of the largest costs associated with aircraft brakes are the initial cost and the maintenance cost to repair and replace the friction material due to wear. The cost of replacement includes the non-flying time of the aircraft. Thus, the initial cost and wear rate of a brake friction material are two critical components in the costs of operating a plane. Because carbon/carbon composite requires long periods of time to make a component (up to three weeks), the cost of this material is quite high. Also, carbon/carbon composite generally displays significantly higher wear due to mechanical abrasion during taxiing versus landing brake operation. This phenomena is probably due in part to the low hardness of the composite.
It would be desirable to provide a brake component which has a low density, high specific heat, and good high temperature properties such as high flexure strength. In particular, and relative to steel and C/C brakes, it is desirable to provide a friction element having the aforementioned characteristics plus stable coefficient of friction and low wear in all modes of operation (i.e., high hardness).